It's true: Colors do change with screen printing. Find out why and what you can do about it.
Low tension will cause the mesh to lift too slowly off the ink on the substrate during the printing cycle. Consequently, the mesh will tend to retain ink and produce a mottled effect on subsequent prints, conditions that cause the color to change. To overcome insufficient screen tension, you must increase the off-contact distance, and once you do that, you must increase squeegee pressure as well. This, in turn, will affect the amount of ink that flows through the mesh during the print stroke, resulting in further color change. For these reasons, always use properly tensioned screens.
Some printers will use the peel function on their press to overcome insufficient mesh tension. This feature raises the frame as the squeegee traverses the stencil, lifting the mesh away from the ink film. The important thing about peel and off-contact settings is that once they have been set for a job, they can't be changed during the print run. This applies with multicolor presses as well as single-color machines. Altering the settings mid-run will change the image size and throw process colors out of registration with one another.
Unless you are printing by hand (in which case you have to use your squeegee to flood the screen), the squeegee carries out just two functions--it brings the stencil into contact with the substrate, and it causes ink to flow into the mesh. If a squeegee is set up incorrectly and does anything else, it will have an adverse effect on the process.
Bearing this in mind and assuming that the angle, pressure, hardness, and dimension of the squeegee are all correct, two additional factors influence the ink deposit. The first is squeegee speed. As a rule of thumb, a fast squeegee will lay down less ink than a slow one. The second and probably most insidious factor is the wear on the squeegee edge that occurs during the print run. The edge of the squeegee will gradually round off over the course of a job, reducing its effective angle of attack. This, in turn, increases the flow of ink into the mesh, resulting in a thicker ink film and a change in color.
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